Money is pouring into Massachusetts' most hotly contested congressional race as Republicans see a chance to peel a seat away from Democrats by ousting Rep. John Tierney.

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The most recent jolt of cash has come in the form of a $350,000 ad buy from the conservative U.S. Chamber of Commerce to help support former state Sen. Richard Tisei, Tierney's sole Republican challenger in the state's 6th Congressional District.

The 30-second ad, running on television and online, portrays Tisei as "an independent voice for Massachusetts" who wants to use free-market solutions to fix health care. It doesn't mention Tisei's party affiliation.

Although the ad is intended to give Tisei a jolt early on in the campaign, it's not the only source of money flowing into the race. Most of it is going directly to the candidates.

Tierney has raised the most of any candidate, Republican or Democratic. Donations to his campaign topped $1.3 million by the end of March.

About 45 percent of Tierney's contributions - $585,400, according to the Federal Election Commission - came in donations from dozens of political action committees representing organized labor, local corporations, lawyers, financial services firms and other interests.

He has also accepted donations from leadership PACs run by fellow Democrats, including U.S. Sen. Elizabeth Warren, former House Speaker Nancy Pelosi, former congressman Barney Frank and U.S. Rep. Joseph Kennedy.

Tisei, by contrast, had raised $753,900 by the end of March, with less than 10 percent, or about $60,600, from PACs.

A campaign aide said Tisei is ahead of his fundraising totals in the 2012 race, when he first challenged Tierney and lost by a narrow margin. Unlike Tierney, who is facing primary opponents, Tisei has no GOP challengers and can focus on the general election in November.

"We're going to have more than enough resources," said Tisei campaign spokesman Charlie Szold, who called spending by outside groups like the U.S. Chamber of Commerce a fact of life in modern campaigns.

"We're just glad that it is a positive expenditure," he said.

One of the surprises in the contest has been the fundraising prowess of Seth Moulton, a Salem Democrat who is challenging Tierney in the party's September primary.

Moulton raised more than $1 million by the end of March, in part by relying on online fundraising tools that have helped him expand his base of supporters well beyond the state.

About 62 percent of Moulton's itemized donations (contributions of $200 or more) came from outside Massachusetts. That compares with about 15 percent of Tierney's itemized donations (not counting PACs) and 11 percent for Tisei.

A Moulton campaign aide said the former U.S. Marine, who was deployed four times to Iraq, has appealed to a broad array of supporters, including those with ties to the Marines. Moulton is also co-founder of Eastern Healthcare Partners, a health care company.

"We're already off to a faster fundraising start this quarter compared to last," Moulton spokesman Scott Ferson said Friday. "His message of the need for change on Washington has resonated."